1. Field of the Invention
The present invention relates generally to an electron-beam gun and more particularly to a gas-shield electron-beam gun for thin-film curing application.
2. Description of the Related Art
For the purpose of increasing the strength and stability of thin films used in semiconductor production, such as insulation film, low-dielectric-constant film and resist film, methods of irradiating an electron beam or ultraviolet light onto the film for the purpose of curing are being studied. In particular, irradiation of an electron beam is effective in improving the properties of low-dielectric-constant film. Two methods, each using a different source, are available for that purpose. One is to use a filament electron-beam gun that heats up a metal and accelerates the generated thermoelectrons using a negative-potential electrode, while the other employs a plasma electron-beam gun that uses plasma electrons.
With a filament electron-beam gun, tungsten or other metal offering high heat resistance is used as a filament, and this filament is heated to a high temperature through resistance heating. In this case, if a reactive gas (such as gaseous oxygen atoms) exists in the space around the filament, the metal of the filament reacts with the gas and deteriorates. The gaseous molecules near the filament are also ionized as they collide with the accelerated electrons. The ionized molecules are then accelerated toward the filament due to the electric field created for acceleration of the electrons, and as a result these molecules collide with the filament. The phenomenon of ion collision further accelerates the deterioration of the filament. A deteriorated-filament part is consumed faster, so filament replacement is required more frequently. Moreover, the continuous operation cycle of electron irradiation treatment becomes shorter, which decreases the productivity of curing. The deteriorated-filament part also causes contamination due to the scattering of metal, which is a serious problem affecting the reliability of semiconductors. In particular, insulation film and low-dielectric-constant film contain large amounts of oxygen, ranging from about 10% to about 70%. Therefore, electron irradiation onto such films causes the oxygen in the film to be outgassed, which can cause severe damage to the electron-beam gun.
To protect the filament part from reactive gases, methods are proposed in which a thin film made of Si or other metal is used to shield an electron-beam gun chamber containing a filament and an acceleration electrode from a chamber containing a target in order to isolate the outgases generated in the target chamber. For example, U.S. Pat. Nos. 5,414,267 and 6,239,543 present vacuum-tube type electron-beam guns, each with a window through which to irradiate the electron beam. This window consists of a thin film made of Si or other metal, which isolates the filament from outgases. In a tube-type electron-beam gun configuration, multiple guns are provided to cover a wide area.
A non-tube apparatus using an electron-beam source with a large area is presented in U.S. Pat. No. 5,003,178. This apparatus is of a plasma electron-beam gun type, not a filament electron-beam gun type. Since in such a apparatus in which a large-area electron-beam source is used it is difficult to provide a large vacuum window while maintaining a high degree of vacuum, the patented apparatus uses a grid-shaped anode between the cathode and the target in the respective vacuum chambers, wherein the distance between the cathode and the anode is adjusted to less than the average free path of electrons, in order to maintain ion plasma while preventing the electrons from reaching the target. However, this configuration is applicable to a plasma electron-beam gun, but not a filament electron-beam gun that must have the electrons reach the target in order to generate energy.